Systems

Key systems design principles

• Isolate the common case [Lampson 84]
  • Prioritize a single use case
  • Identify read versus write workloads and optimize accordingly
  • Identify need for distributed computing versus single node (LRPC) ecure
• Think about consistency requirements carefully. Relaxing consistency can provide increased availability (Coda) or better performance
• Indirection can simplify design and decouple evolution of upper and lower layers of a stack
  • Examples: IP, OS, VMM, LLVM IR, SQL
• End-to-end arguments
  • Carefully think about implementing functionality at a lower layer if it must be implemented at a higher layer as well
  • Add it at a lower level if performance improves and does not compromise performance for other workloads that don't need it
  • RISC, IP, exokernels
• Specialization
  • Hold other dimensions of design constant while improving single attribute
  • Leverage semantics for one key workload (common case)
  • SQL focuses on structured data
  • CRDT focuses on a few commutative operations
  • Google FS optimizes append-only workloads
• Simplicity (Occam's)
  • easier to add complexity later, but hard to remove it
  • allows for fast iteration
• Get correctness first and then optimize later
• Why do systems fail (Stonebraker)?
• Excessive complexity
• Unecessary extensions and functionality
• No compelling use case that is widely accepted

Systems tradeoffs

• Functionality vs performance vs x
  • x can be usability, consistency, safety, security
• Performance vs portability
• Latency vs bandwidth vs fault-tolerance requirements [Clark 95]
• Fine-grained vs coarse-grained
  • coarse-grained provides performance but is more complex and less s
• Hardware vs software support
• Interactive vs batch systems

Ballpark latency numbers (from Jeff Dean)

L1 cache reference ......................... 0.5 ns
Branch mispredict ............................ 5 ns
L2 cache reference ........................... 7 ns
Mutex lock/unlock ........................... 25 ns
Main memory reference ...................... 100 ns             
Compress 1K bytes with Zippy ............. 3,000 ns  =   3 µs
Send 2K bytes over 1 Gbps network ....... 20,000 ns  =  20 µs
SSD random read ........................ 150,000 ns  = 150 µs
Read 1 MB sequentially from memory ..... 250,000 ns  = 250 µs
Round trip within same datacenter ...... 500,000 ns  = 0.5 ms
Read 1 MB sequentially from SSD* ..... 1,000,000 ns  =   1 ms
Disk seek ........................... 10,000,000 ns  =  10 ms
Read 1 MB sequentially from disk .... 20,000,000 ns  =  20 ms
Send packet CA->Netherlands->CA .... 150,000,000 ns  = 150 ms